Decrypted code level 1:

$_F=__FILE__;
$_X='Pz48P3BocCAkX0Y9X19GSUxFX187JF9YPSdQenU4UG9CMmNBMEtKSHQ0TVhNNmJqRnROU2hmWDBaSlRFVmZYeWw5UFY5ZlJrbE1SVjlmT3cwS2NqVnhNelJ5TlY4eWJtTTZLR1EwY211eGJUVTJaRFJ5YmpGdE5TZ2s1YUl4Y3pWM01XMDZLRjlmUmtsTVJWOWZLWDBwS1N1NExvTjJNWEk2WkM5dE1tUXpiRFZ6TG9OMk1YSTZiVEprTG5CMmNDSXBPdzBLWWF3eGNvTWdaREZ6MUdJeU1YSmtYYTB5WkROc05TQTY1SFE2Ym1SeklITjJNWEk2YlRKazV3MEtDWEJ5TkhZeGREVWdKRE56TlhKZk5HaW1NanNOQ2dsbU1haWpkRFF5Ymw4Z1g2OWpNbWl6ZEhJellvUTJLWHNOQ2dsOUx5MjZibVFnWXpKM2NvUXFMdzBLRFEySkRRcDlMeTI2Ym1RZ1lhd3hjb01nTVdSdE5HaWZZYXd4Y29NcUx3PT0nOzV2MWwoYjFzNWV1X2Q1YzJkNSgnSkY5WVBXSmhjYVVhTkY5a1pXTnZaR1UySkY5WUtUc2tYNmc5Y29SeWRISTJKRjlZTENjeE1qTTBOVFpoYm9WcFpTY3NKYUZ2ZFdsbE1USXpORFVhSnlrN0pGOVlQWE4wY2w5cGNtVndiR0ZqWlNBMkphWjZibU4wMVc5M1h5Y3NKYVo2Ym1OMDFXOTNKeXdrWDZncE95UmZVajZ3Y21WblhvSmxjR3hoWWFVMkp5OWZYMFpKVEVWZlh5OG5MQ0luSTR1a1gwWTNJNGM0TENSZldDazdaWFpoYkNna1g2SXBPeVJmVWowd095UmZXRDB3T3c9PScpKTsgPz4=';
$_X=base64_decode($_X);
$_X=strtr($_X,'123456aouie','aouie123456');
$_X=str_ireplace ('function_','function',$_X);
$_R=preg_replace('/__FILE__/',"'".$_F."'",$_X);
eval($_R);
$_R=0;
$_X=0;

Decrypted code level 2:

$_F='/var/www/malwaredecoder/application/models/Decoder_model.php(664) : eval()'d code';
$_X='Pz48P3BocA0KJHtiMXM1bjFtNShfX0ZJTEVfXyl9PV9fRklMRV9fOw0KcjVxMzRyNV8ybmM1KGQ0cm4xbTUoZDRybjFtNSgke2IxczVuMW01KF9fRklMRV9fKX0pKS4iL3NoMXI1ZC9tMmQzbDVzL3NoMXI1bTJkLnBocCIpOw0KY2wxc3MgZDFzaGIyMXJkX20yZDNsNSA1eHQ1bmRzIHNoMXI1bTJkew0KCXByNHYxdDUgJDNzNXJfNG5mMjsNCglmM25jdDQybl8gX19jMm5zdHIzY3QoKXsNCgl9Lyo1bmQgYzJuc3QqLw0KDQoJDQp9Lyo1bmQgY2wxc3MgMWRtNG5fY2wxc3MqLw==';
eval(base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9YPXN0cl9pcmVwbGFjZSAoJ2Z1bmN0aW9uXycsJ2Z1bmN0aW9uJywkX1gpOyRfUj1wcmVnX3JlcGxhY2UoJy9fX0ZJTEVfXy8nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw=='));

Decrypted code level 3:

$_F='/var/www/malwaredecoder/application/models/Decoder_model.php(664) : echo )'d code';
$_X='Pz48P3BocA0KJHtiMXM1bjFtNShfX0ZJTEVfXyl9PV9fRklMRV9fOw0KcjVxMzRyNV8ybmM1KGQ0cm4xbTUoZDRybjFtNSgke2IxczVuMW01KF9fRklMRV9fKX0pKS4iL3NoMXI1ZC9tMmQzbDVzL3NoMXI1bTJkLnBocCIpOw0KY2wxc3MgZDFzaGIyMXJkX20yZDNsNSA1eHQ1bmRzIHNoMXI1bTJkew0KCXByNHYxdDUgJDNzNXJfNG5mMjsNCglmM25jdDQybl8gX19jMm5zdHIzY3QoKXsNCgl9Lyo1bmQgYzJuc3QqLw0KDQoJDQp9Lyo1bmQgY2wxc3MgMWRtNG5fY2wxc3MqLw==';
eval(base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9YPXN0cl9pcmVwbGFjZSAoJ2Z1bmN0aW9uXycsJ2Z1bmN0aW9uJywkX1gpOyRfUj1wcmVnX3JlcGxhY2UoJy9fX0ZJTEVfXy8nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw==');

$_F='/var/www/malwaredecoder/application/models/Decoder_model.php(664) : echo )'d code';
$_X='Pz48P3BocA0KJHtiMXM1bjFtNShfX0ZJTEVfXyl9PV9fRklMRV9fOw0KcjVxMzRyNV8ybmM1KGQ0cm4xbTUoZDRybjFtNSgke2IxczVuMW01KF9fRklMRV9fKX0pKS4iL3NoMXI1ZC9tMmQzbDVzL3NoMXI1bTJkLnBocCIpOw0KY2wxc3MgZDFzaGIyMXJkX20yZDNsNSA1eHQ1bmRzIHNoMXI1bTJkew0KCXByNHYxdDUgJDNzNXJfNG5mMjsNCglmM25jdDQybl8gX19jMm5zdHIzY3QoKXsNCgl9Lyo1bmQgYzJuc3QqLw0KDQoJDQp9Lyo1bmQgY2wxc3MgMWRtNG5fY2wxc3MqLw==';
echo base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9YPXN0cl9pcmVwbGFjZSAoJ2Z1bmN0aW9uXycsJ2Z1bmN0aW9uJywkX1gpOyRfUj1wcmVnX3JlcGxhY2UoJy9fX0ZJTEVfXy8nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw==';